In semiconductor manufacturing, plasma reactor chambers are used to remove or deposit material on a semiconductor substrate in the process of making integrated circuit (IC) devices. A key factor in obtaining the highest yield and overall quality of ICs is the uniformity of the etching and deposition processes.
A problem that has plagued prior art plasma reactors is the control of the plasma to obtain uniform etching and deposition. In plasma reactors, the degree of etch or deposition uniformity is determined by the design of the overall system, and in particular the design of the electrodes used to create the plasma in the interior of the reactor chamber.
One approach to improving etch and deposition uniformity has been to use a segmented electrode. An exemplary prior art segmented electrode 700 is shown in FIG. 1. Segmented electrode 700 includes separate thick conducting electrode segments 704 separated by an insulator 710, which is a single-piece shaped like a wheel with a hub, a rim, and spokes, and is housed in a chamber frame 714. The design of segmented electrode 700 is such that electrode segments 704 contact a vacuum region 720 on one side, and atmospheric pressure region 724 on the other side. This puts electrode segments 704 directly in contact with the plasma formed in vacuum region 720. In addition, each segment 704 of segmented electrode 700 has numerous conduits 734 through which a cooling fluid 740 must flow to cool the segments during operation. Further, segmented electrode 700 includes numerous gas feed lines 744 for introducing gas into vacuum region 720. Moreover, numerous seals 750 are required between insulator 710 and electrode segments 704 to isolate vacuum region 720 from atmospheric pressure region 724. The need for multiple gas lines and multiple sets of cooling lines significantly complicates the electrode and chamber design, and makes for a complex plasma reactor apparatus that is more susceptible to failures.
With continuing reference to FIG. 1, insulator 710 serves to separate electrode segments 704 to minimize inter-electrode capacitance and cross talk. Typically, insulator 710 needs to be cut or otherwise formed into a complex shape, which is expensive and difficult to manufacture. In addition, insulator 710 is typically fragile and thus prone to breaking because of the small size of the critical dimensions of the insulator. Even once it has been made, the insulator is prone to cracking because of mechanical stress that builds up at the relatively sharp corners. In addition, despite improvements in plasma uniformity achieved with prior art segmented electrodes, sharp local changes in etch or deposition rate over the wafer, i.e., the “electrode pattern imprint”, still occurs.
There are several patents pertaining to segmented electrodes for use in plasma etch apparatus. For example, U.S. Pat. No. 5,733,511, “Power distribution for multiple electrode plasma systems using quarter wavelength transmission lines”, (the '511 patent) describes a multiple electrode plasma reactor power splitter and delivery system to provide balanced power to a plurality of powered electrodes by utilizing the properties of quarter wave length transmission lines. Each electrode is supplied power by a separate (2N+1)λ/4 wavelength cable, where N=0, 1, 2 . . . , connected to a common point at a load match network's output. The impedance transformation properties of these lines are also employed to convert the plasma load to one that is more efficiently matched into by a standard network. Also disclosed is a technique of splitting a single large active electrode into smaller active electrodes powered by the above distribution scheme in order to achieve maximum uniformity of the reactive plasma throughout the working volume. However, a shortcoming of the apparatus described in the '511 patent is that the electrode segments are not driven by separate RF power sources, but the λ/4 cables are all connected to a common point at the match network output, so that there is no means provided for control of individual segments. Also, according to the Figures in '511 patent, it appears that the segment electrodes are physically separate of each other, like that shown in FIG. 1 herein, which leads to an excessively complex design and susceptibility to failure.
U.S. Pat. No. 4,885,074, “Plasma reactor having segmented electrodes,” (the '074 patent) describes a plasma reactor for generating a uniform field of energized gas for plasma processing. A mechanism for mounting a workpiece is disposed within the reactor chamber so that a workpiece can be exposed to energized gas. A first electrode in the chamber is positioned in operative relationship to the workpiece mounting mechanism and a second electrode within the reactor is positioned to at least partially surround the first electrode. However, a shortcoming of the '074 patent is that the segment electrodes are ring shaped and do not provide azimuthal (circumferential) control of uniformity. Accordingly, an off-center peak etch-rate cannot be corrected with the electrode geometry of the '074 patent.
U.S. Pat. No. 5,006,760, “Capacitive Feed for Plasma Reactor,” (the '760), describes a capacitive feed for the lower electrode in a parallel plate plasma reactor. One plate of the capacitor comprises the lower electrode or a contact to the lower electrode. The other plate of the capacitor comprises an annular member insulated from the lower electrode, or the contact. There are no RF connections directly to the lower electrode. However, the movable electrode is not segmented and is not for the purpose of altering the plasma density profile to account for etch or deposition non-uniformity.
Accordingly, it would be much preferred to have a way of modifying the plasma density profile in a plasma reactor to achieve an improved etch, deposition or other plasma process uniformity without the design complexities and shortcomings associated with present-day segmented electrodes. In addition, it would be much preferred to have a way of controlling the plasma density profile to achieve a desired effect, even if it means creating a non-uniform plasma density profile. These non-uniform plasma density profiles are often required to accommodate non-uniformity created by previous non-uniform wafer processing steps.